Primer-dimer problems

Greetings, All.
I am getting inconsistent results with a primer set which
should give me a single band of about 550 bp. One thing I have
noticed is the existence of primer-dimers and and primer ladders
(up to 3 bands) in the lanes where I get little or no specific
product.
I know that primer-dimers can be favored when the
annealing temperature is too low or the Taq or primer
concentrations are too high. Primer Tms are both 53.7 ; product
Tm (calculated) is 77.9 . I saw no product with an annealing
temp of 65 but a nice band at 60 . I routinely use 0.4 units of
Taq polymerase (in the form of TaqStart) and 25 pmol of each
primer per 50 æl reaction.
What else can favor primer-dimer formation? I'm thinking
that if I can push the conditions away from primer-dimers I might
get more product.
--
Chris Zarow, Ph.D
University of Southern California
Rancho Los Amigos Medical Center
zarow at molbio.usc.edu